Prior Art Counterbalance Valves
Counterbalance (CB) valves are applied in various hydraulic circuits and are used to hold, capture, or control the motion of loads. Essentially, CB valves are modulating devices that allow free flow from an inlet valve port to a load port and then block reverse flow until a load pressure, pilot pressure, or combination of load and pilot pressure open the valve. Modern CB valves control loads and minimize instability (e.g., chattering) in machines. As a result, CB valves are frequently incorporated in positioning circuits, load-holding and purging circuits, and regenerative circuits. Examples of machines that utilize CB valves are boom lifts, forklifts, industrial drills, and excavators, among many other machines.
The modulation of conventional CB valves is a function of both load pressure and pilot pressure. This characteristic ensures loads must be driven from a fluid power source, even when loads are overrunning. FIG. 3 graphically depicts how pilot pressure decreases as load pressures increases. In particular, the graph of FIG. 3 illustrates a prior art CB valve in which pilot pressure I changes as load pressure II rises steadily with time to a predetermined maximum setting III. On the other hand, pilot pressure would rise if load pressure fell steadily over time.
FIG. 1 illustrates a prior art, three port, pilot-to-open CB valve 1001 having a valve body 1038, a load port 1004, a valve port 1008, and a pilot port 1012. In operation, CB valve 1001 serves as a counterbalance and holds a load at load port 1004. If the load pressure applied at load port 1004 exceeds the spring force created by springs 1020, then piston 1032 moves axially to compress springs 1020. Such axial movement of the piston to an open position creates a flow path between load port 1004 and valve port 1008, or put differently, it provides a relief function. Because having only a relief function is inefficient, a pilot function is also provided. The pilot function allows a user to apply pilot pressure to open a flow path between the load port 1004 and the valve port 1008. As a result, pressure at either the load port or the pilot port can be used to open a flow path between the load port and the valve port. A combination of pressure at the load port and the pilot port will also open a flow path.
Traditional CB valves have a fixed or adjustable setting. Fixed setting valves are typically set by the manufacturer and are not adjustable. Adjustable CB valves feature adjustment screws so the setting can be changed in the field. While settings can be set in the field, settings are not typically changed when the valves are being operated. CB valves with adjustment screws are illustrated in FIGS. 1 and 2. Adjusting CB valves generally involves rotating adjusting screw 1024 when the CB valve is not in operation. Rotating adjusting screw 1024 changes the compression of springs 1020, thus changing the setting of the CB valve. More specifically, when springs 1020 are in an uncompressed position, the CB valve allows piston 1032 to slide to an open position at a low load pressure. In contrast, when the springs are adjusted to a compressed position, a greater load pressure is necessary to move the piston to an open position.
FIG. 2 illustrates a prior art, four port, pilot-to-open CB valve 1036. CB valve 1036 is vented to eliminate the effect back pressure on the valve port has on the operation of the valve. In particular, CB valve 1036 is provided with drain or vent port 1016 that eliminates sensitivity to back pressure.
Drawbacks of Prior Art Counterbalance Valves
Although CB valves have highly desirable qualities in the hold, capture, and motion control of loads in hydraulic circuits, CB valves also have drawbacks. CB valves must be set for the maximum load that a machine must control. One significant drawback is increased energy consumption in machines that utilize conventional CB valves, when load pressures vary significantly over the operating cycle. Another drawback is increased wear in machines and machine components when high pilot pressures are frequently needed to control light loads. Although energy efficiency and wear and tear have long been important considerations in machine design, the current economic, environmental, and political atmosphere has led to an extraordinary demand for innovations that improve efficiency and longevity of machines, including machines that incorporate CB valves.
Both increased energy consumption and wear of machines operating with traditional CB valves are owed, at least in part, to CB setting and pilot ratio. First, for control and safety purposes, to maintain control of maximum anticipated loads, CB valves are typically set above the maximum load pressure that could be generated. In many machines, however, maximum loads may be encountered infrequently. That is, the majority of time during which a CB valve is in operation, the load pressure is some amount less than the CB setting. During that time, an increase in pilot pressure is necessary to compensate for the difference between the valve setting and the load pressure. Second, and of significant importance, up to half of the working cycle of many CB valves is dedicated to the control of nominal loads, where machines are unloaded. This means that for up to half of its working cycle, and sometimes more, a CB valve may require use of full or high pilot pressure.
By way of example, a forklift may be used to raise a full pallet, where it is unloaded and then lowered completely empty. Lowering the empty fork typically requires high pilot pressure. Generating high pilot pressure, of course, necessitates the use of high horsepower and energy. In a forklift, significant pilot pressure, and energy, may be required when lowering any load less than a full load.
As a result, there is a real need for devices that provide the hold, capture, and motion control benefits of current CB valves while improving upon one or more performance characteristics of current CB valves.